For a long time people in Hungary have enjoyed the liberty to go shopping whatever they want to, also on Sundays. That is why even the Austrians would cross the borders and come to Hungary on Sundays to go shopping, while all shops in Vienna and elsewhere in Austria are closed on Sundays. But, a new era has started in Hungary, because now shops will stay closed on Sundays here as well!
This sounds terrifying to many, both locals and tourists. In other words, if you plan on visit a shopping mall on Sundays in Budapest you will find mostly everything to be closed, except from some coffee houses and cinemas. The only place you can actually go shopping as of know is in the shopping street of Budapest, where shops are supposed to be allowed to keep open because it is a World Heritage site, and there are some exceptions if the actual owner of the shop actually works in the shop on that day.
The first Sunday where this will actually be taken into action is this upcoming Sunday (March 22nd). If you plan on doing a trip to Szentendre this will probably be causing no problems, because shops there will stay open as in most cases the actual owners actually work in their own shops, and because to keep closed on Sundays would make them loose so much money, most owners will actually do the work themselves, if that is what it takes to be allowed to open their small and cute shops!
So be aware of this when you come to do shopping in Budapest!